As a general rule, free-space optical communication systems have a number of advantages over other communication systems. They offer a level of security that is not present in a radiofrequency system; the optical communication signal beam can be highly collimated and directed at the intended optical receiver. In addition, optical communication links can transmit at very high data rates.
Former methods to produce such free-space optical communications systems involved the use of multiple optical receivers and high powered, mechanically steered laser optical transmitters. Since the laser beam could not be electronically steered, its direction was controlled by a mechanical gimbal system, for example. The receivers of the optical communication links in former approaches were typically based on photodiodes with optics to gather the light from the optical transmitter.
This approach is fine when the channel is stable and the platforms are not moving quickly. However, in the presence of disturbances such as atmospheric disturbance, or in the presence of motion such as encountered in mobile applications, the optical receiver must also be mechanically steered to maintain alignment to the transmitter. Further, due to the atmosphere, light from the laser is refracted in directions that do not fall on the optical receiver. Thus, mechanical steering is required at both ends of the channel. Further, multiple receivers are often required for redundancy to ensure that the optical communications link is robust against movement and disturbances.